1. Field of the Invention
The present invention relates to a method and a device for transmitting data between at least two users of a bus system, the transmitted data frames having a logic structure according to the ISO 11898-1 CAN standard, and at least two additional short bits being inserted within the temporal bit length of at least some of the CAN bits, such that for at least one of the two possible values of the current CAN bit, the first one of the additional bits inserted into this CAN bit is transmitted using a bus level that is the opposite of this value.
2. Description of the Related Art
For example, from the ISO standard family 11898, the Controller Area Network (CAN) as well as a broadening of the CAN referred to as “Time-Triggered CAN” (TTCAN) is known. The media access control method used in the CAN is based on a bit-wise arbitration. In the bit-wise arbitration, multiple user stations are simultaneously able to transmit data via the channel of the bus system, without interfering with the data transmission in so doing. Furthermore, the user stations are able to ascertain the logical state (0 or 1) of the channel while transmitting a bit over the channel. If a value of the transmitted bit fails to correspond to the ascertained logical state of the channel, the user station terminates the access to the channel. In CAN, the bit-wise arbitration is usually carried out in an arbitration field within a data frame that is to be transmitted via the channel. After a user station has sent the arbitration field to the channel in its entirety, it knows that it has exclusive access to the channel. Consequently, the end of the transmission of the arbitration field corresponds to a beginning of an enable interval, within which the user station is able to use the channel exclusively. According to the protocol specification of the CAN, other user stations may not access the channel, that is, send data to the channel, until the sending user station has transmitted a checksum field (CRC field) of the data frame. Thus, an end point of the transmission of the CRC field corresponds to the end of the enable interval.
The bit-wise arbitration allows a non-destructive transmission of the data frame over the channel. Because of this, excellent real-time properties of the CAN come about, whereas media access control methods, in which the data frame sent by a user station may be destroyed during transmission via the channel because of a collision with a further data frame sent by another station have a clearly more unfavorable real-time behavior since the collision and the thereby required retransmission of the data frame causes a delay in the data transmission.
The protocols of the CAN are particularly suitable for transmitting short messages under real-time conditions. If larger data blocks are to be transmitted via a CAN domain, the relatively low bit rate of the channel becomes a limiting factor. In order to assure the correct functioning of the bit-wise arbitration, it is necessary to observe a minimum duration for the transmission of a bit during the arbitration; this duration is first of all a function of the extension of the bus system, the signal propagation speed on the channel and intrinsic processing times in the interface modules of the bus users, because all bus users must have a uniform image of the state of the bus (0 or 1) and equal priority access to the state of the bus. Therefore, the bit rate cannot readily be increased by reducing the duration of the individual bits.
In order to nevertheless be able to transmit, with sufficient speed, a relatively large data block required for the programming of a control unit via a communication interface that is actually provided for connection to a CAN domain, the published German patent application document DE 101 53 085 A1 proposes a temporary switchover of the communication interface for transmission of the data block to another communication mode, in which no bit-wise arbitration is performed, and a relatively high bit rate is possible as a result. However, the communication with the protocols of the CAN has to be interrupted for a certain period of time in this case. If, for example, the operation of the bus system cannot be resumed again according to the CAN protocols because of an error, there is a failure of the bus system. In addition, the transmission of a relatively large data block will cause a considerable delay in the subsequent transmissions to be undertaken according to the CAN protocols, so that the real-time properties of the CAN are impaired.
Published German patent application document DE 103 11 395 A1 describes a system in which the asynchronous, serial communication is alternatively able to take place via an asymmetrical physical protocol or via the symmetrical physical CAN protocol, and because of this, a higher data transmission rate or data transmission security is achievable for the asynchronous communication.
In the “Proceedings of Design, Automation and Test in Europe (DATE 2009)”, IEEE Computer Society, Nice, France, pp. 1088-1093, Apr. 20-24, 2009, Ziermann et al. propose another approach for obtaining a higher data-transmission capacity: Within an enable interval, designated as “S-zone”, of the CAN data frame, in which a user station uses the channel exclusively, additional short bits are added to the individual CAN bits within a so-called “gray zone”.
It becomes obvious from the cited documents that the related art does not supply satisfactory results from every point of view.
The method proposed by Ziermann et al. envisions a mixed operation with bus users operating according to the CAN standard, and bus users utilizing the newly proposed communication method. The data to be transmitted in addition are split into packets of additional short bits. The packets of additional short bits are inserted into the individual CAN bits in the gray zone in each case, which is set up in such a way that it has ended prior to the scanning instant of the CAN bit. As a result, the bus users operating according to the CAN standard ideally do not notice the insertion of the short bits.
Bus users operating according to the CAN standard resynchronize their internal bit timing with the aid of the signal flanks from recessive to dominant which are applied at the data bus (see ISO 11898-1, ch. 12.4.2). The flanks created by the insertion of the additional bits in the gray zone may lead to interference in this resynchronization process or the synchronicity of the bus users and thus to falsifications or interruptions of the data communication.
Therefore, it is an object of the present invention to describe a method by which data frames with additionally inserted data are able to be transmitted in a CAN network such that the synchronicity of the bus users, which is required for the arbitration, is maintained by suitable measures.